The present invention is directed to magnetic coupling driven pumps and more particularly to barrier cans which prevent any leakage between the inner magnet attached to the pump, the outer magnet attached to the pump drive or motor and most particularly to a sealless non-metallic composite barrier can for use with magnetic drive pumps.
Pumps having magnetic couplings are typically used in the processing of fluids, such as, for example, fuel oils, lubricating oils, asphalt, crude oil, synthetic fiber dopes and melts, vegetable and animal oils, hydraulic fluids, cutting oils, coolants, jet fuel, molasses and syrup, composite fuels, fire-resistant hydraulic fluids and compatible chemicals and include but are not limited to axial flow multi-rotor positive displacement screw pumps. Axial flow multi-rotor positive displacement screw pumps have precision-ground screws meshing within a close fitting housing to deliver non-pulsating flow quietly and efficiently. The opposed idler rotors act as rotating seals confining the fluid in a secession of closures or stages. The idler rotors are in rolling contact with the central power rotor and are free to float in their respective housing bores on a hydrodynamic oil film. There are no radial bending loads. Axial hydraulic forces on the rotor's set are balanced, eliminating any need for thrust bearing.
Driving sealless rotary screw pumps with magnetic coupling components is known to provide benefits not found in other sealless pump products. The magnetic coupling sealless pumps feature a synchronous drive (motor speed=drive speed) with permanent rare earth magnets on both the inner and outer coupling halves. The magnetic attraction firmly locks the drive rotor together with the motor drive shaft give consistent torque transmission at any speed.
The axial flow path and low inertia rotors, inherent to rotary screw pumps, generally allow operation at higher speeds than other types of rotary pumps. Because of this capability, rotary screw pumps take full horsepower advantage of the torque characteristics of magnetic coupling drives.
During pump operation, the changing magnetic flux produced by the rotating coupling induces voltages within the barrier can material. In an electrically conducting barrier can material, the induced voltages produce localized eddy currents. Power, in the form i.sup.2 R=V.sup.2 /R where R is resistance, appears as heat on the surface of the cylindrical portion of the conducting can material. The amount of heat generated intensifies with the square of speed, as rotational speed (flux change rate) is increased, thereby limiting the maximum operating speed of the pump.
It is known that excessive heat affects the components of the magnetic coupling adversely. As heat is convected in the coupling area, magnetic field strength is reduced proportionately to the increasing temperature of the permanent magnets. Temperatures above about 150.degree. C. are known to permanently destroy Neodymium-Iron-Boron magnets and temperatures above 250.degree. C. are known to permanently destroy samarium-cobalt magnets.
It is also known that a magnetic coupling which produces excess heat is less efficient than a standard mechanical coupling. A given amount of energy is supplied for operating the pump at a certain rotational speed and that energy is then converted by the motor into work. In a magnetic coupling utilizing a conducting barrier can material, energy lost to excess heat results in less work being accomplished for a given amount of input energy.
It is further known that heat generated in the coupling is convected into the fluid being pumped or pumpage. As a result, the viscosity of the pumpage may be changed thereby. A viscosity change in the pumpage has been known to trigger problems in the pumping system or affect the usefulness of the pumpage. Excessive heat generated by the eddy currents have been known to scald, ignite, or otherwise ruin the pumpage or pumpage additives.
State of the art magnetic drive pumps employing barrier cans made of conductive material are designed to prevent rapid heating by diverting pump flow for continuous flushing of the barrier and inner magnet assembly areas. Lower speeds and viscosities are typically specified for magnetic drive pumps than for mechanically coupled pumps. Oversizing the magnetic drive pump drive motor to compensate for horsepower loss has become a common practice. However, each of these measures detracts from pump efficiency, increases manufacturing costs and, thus, increases the cost of the magnetic drive pumps to the end users.
Heat generated by eddy currents has been reduced by manufacturing barrier cans from nonmagnetic and high electrical volume resistivity steel. However, high levels of heat are still generated in the magnetic couplings having such barrier cans. Some further heat reduction has been achieved in the non-magnetic steel barrier can by producing the can in sections and laminations, as shown in the coupling design described in U.S. Pat. No. 4,896,064.
The cans described therein required the use of gasketing and bonding adhesives to achieve leak proof operation and electrical insulation between laminations. The presence of gaskets and bonding adhesive materials limit the barrier can chemical compatibility with pumped fluids in that, if incompatible, the gaskets and bordering material may contaminate the pumpage.
Since each joint in the barrier can of the U.S. Pat. No. 4,896,064 patent is a potential leak path, it would not be acceptable in high pressure and/or high temperature pump environments. A magnetic coupling utilizing the type of barrier can described in the '064 patent must also be designed to accommodate the substantial wall thickness caused by the laminations, gasketing, and bonding.
As is known, thicker can walls increase the gap between the inner and outer magnet assembly assemblies. This gap increase dramatically reduces coupling efficiency and, thus, requires stronger inner and outer magnets to produce the same torque capability as magnetic coupling having smaller gaps between the inner and outer magnet assembly. Thus, the materials, assembly, and system requirements make the barrier can described in the '064 patent a cost prohibitive option.
Although it is presently possible to eliminate heat generation due to eddy currents through the use of non-metallic barrier cans, state of the art metallic barrier cans hold strength and cost advantages over state of the art polymer and ceramic barrier cans. Polymer cans are typically suitable only for low pressure applications and lack dimensional stability during pumpage temperature fluctuations. The presently known ceramic barrier cans must be first molded with a relatively thick wall and then machined on the inner diameter to achieve desirable dimensions and tolerances. It is the post-mold machining of ceramic cans during manufacturing that makes them unacceptable from a cost stand point. Additionally, the thick walls increase the air gap between the magnets resulting in lower pump efficiencies.
An additional problem with conventional magnet coupling is that detection of the temperature in the magnetic coupling has proven difficult. Magnetic coupling temperatures are typically acquired by means of a temperature probe placed straight through the pump bracket to the area of the barrier can near the flange. Temperatures in this area are not as high as temperatures in the areas near the closed end of the barrier can. A probe placed in this area typically has failed to detect temperature fluctuations inside the magnetic coupling that could lead to magnetic coupling failure. The result of such conventional probe placement has been a coupling temperature monitoring device which does not accurately detect the magnetic coupling operation temperature near the closed end of the barrier can. More accurate detection of the magnetic coupling temperature in the vicinity of the cans can be achieved by locating the temperature detecting device nearer the closed end of the barrier can. The area near the closed end requires affixing wiring for the device along the exterior of the barrier can and through the flange at the open end of the barrier can. Since this wire path is vulnerable to damage during handling and operation, it has not been considered a viable design.
Thus, there is a need for a magnetic coupling pump which overcomes the deficiencies listed above. Specifically, such a pump should include a magnetic coupling which produces significantly less heat from eddy currents; should significantly reduce energy lost to excessive heat, thereby improving the amount of energy available to accomplish work; should reduce the amount of heat which is convected into the pumpage; should make the magnetic coupling type pumps more competitive with mechanical drive type pumps, thereby reducing, if not eliminating, the requirement to oversize the magnetic drive pump motors in order to compensate for horsepower loss due to the excessive heat losses; should decrease the manufacturing costs of the magnetic coupling; should reduce, if not fully eliminate, gasketing and bonding adhesives and related material which may deteriorate and contaminate the pumpage; should maintain as small a gap as possible between the inner and the outer magnet assemblies; should reduce the size of the inner and outer magnets, such that, smaller inner and outer magnets produce the same torque capability as was produced by the magnetic coupling having conventional cans; should provide for improved sealless operations and should provide for the placement of temperature probe in various positions within a magnetic coupling can in order to more accurately sense or detect the temperature of the magnetic coupling near the closed end thereof.